1. Field of the Invention
The present invention relates to a method of etching metallic films for semiconductor devices, and more particularly, to a method of etching metallic films using plasma formed by electrostatic shielded radio frequency (ESRF) inductive-coupled plasma sources.
2. Description of the Related Art
Generally, during the fabrication of semiconductor devices, an etching process is performed in order to form patterns on a film deposited on a semiconductor substrate. The parameters of the etching process depend on the characteristics of the materials and structures designed for the semiconductor device. With more highly integrated semiconductor devices, the pitch, i.e., the horizontal spacing, of structures has become smaller, while the number of layers has increased, leading to the need for creating trenches through more layers. That is, there is a need for a decreased minimum pitch (critical dimension) and an increased aspect ratio (i.e., the ratio of depth of a trench to width of the trench) of the patterns formed in the films for integration scales beyond large scale integration (LSI). Productivity of the fabrication process also depends on etching rate, i.e., the depth of a film that can be etched in a unit of time.
To date, a variety of etching methods have evolved--from wet etching using chemicals, to several dry etching methods including using pure plasmas, reactive ion etching (RIE), magnetically enhanced reactive ion etching (MERIE), electron cyclotron resonance (ECR) plasmas, and electrostatic shielded radio frequency (ESRF) inductive coupled plasma sources.
The etching rates of most of these methods are limited by the depletion of etching agents in the vicinity of a film being etched as the etching progresses, a phenomenon called micro-loading. Micro-loading can be greatly affected by the rate at which reactive etching agents are supplied to the vicinity of the film and the rate at which products of the reaction are carried away.
Plasmas are generated in plasma chambers in which electrons of a source gas, such as air at normal pressure, are excited by radio frequency (RF) high voltages applied across two electrodes or induced by RF electric current flow in a coil. The excited electrons then collide with the molecules of an etching gas, i.e., a process gas, to produce ions and radicals. The ions and radicals then etch the film deposited on the substrate.
Pure plasma etching generates chemical agents including ions and radicals from a plasma of excited electrons so that the ions and radicals can react chemically with the film deposited on the substrate. The effectiveness of this type of etching depends greatly on the material being etched and so provides a high degree of selectivity, like chemical etching. For many materials this pure plasma etching proceeds in all directions, i.e., the etching is isotropic.
Reactive ion etching (RIE) further accelerates the ions and radicals toward the film to increase the kinetic energy of collisions between the reactive ions and radicals and the molecules of the film. This ionic bombardment increases the effectiveness of etching, especially in the direction of bombardment, generally perpendicular to the film. Therefore more directional, i.e. more anisotropic, etching occurs than with pure plasma etching. RIE also tends to depend less on the material of the film, i.e., RIE provides less selectivity than pure plasma etching. In a typical RIE device the RF is at 13.56 MegaHertz (MHz), and a capacitor in the grounding line allows a negative charge to accumulate on the walls of the chamber and the on the wafer. The negative charge repels electrons in an adjacent region called a sheath, and accelerates positive ions and radicals toward the walls and wafer.
Electron densities sufficient to produce ions and radicals can be achieved at low pressures by introducing a magnetic field that constrain the electrons to move in circular or helical trajectories as in MERIE. In ECR plasmas, the magnetic field strength is selected to produce a Lorenz force that causes circular or helical electron trajectories with a cyclotron frequency that matches the RF applied to the electrodes. In ECR plasmas, efficient coupling is obtained between the power applied to the electrodes and the energy given to the electrons. This is called resonant energy coupling, and usually uses a RF at 2.45 GigaHertz (GHz).
Independent plasma generation and ion acceleration is achieved in the inductive coupled plasma sources. In these devices, electrons are excited to form a plasma by an RF magnetic field induced by applying RF power to a helical coil. The plasma is constrained in a plasma region by the RF magnetic filed. A process gas is introduced into the chamber and is ionized by the plasma. A separate bias voltage is applied to the wafer to accelerate the ions and radicals into the wafer at steep angles. To reduce acceleration of plasma and ions and radicals into the walls of the chamber by secondary electric fields produced by the coil as a byproduct of the RF magnetic field, an ESRF inductive coupled plasma source provides a slotted cylindrical conductor around the wall of the chamber as an electrostatic shield. The electrostatic shield prevents the accumulation of charge on the chamber walls by shorting the fields directed perpendicular to the wall, which cause a capacitive coupling, to ground. A solid electrostatic shield would also block the propagation of RF electromagnetic waves into the chamber. However those waves are needed to establish the axial RF magnetic field. Therefore the shield is slotted to allow the RF electric fields to propagate that support the axial RF magnetic field desired, and that block the waves that provide perpendicular RF magnetic fields. In this arrangement ions and radicals can be accelerated into the wafer with less acceleration into the walls of the chamber, and the ions and radicals can be accelerated with a power independent of the plasma generating RF power. As a result low energy ion bombardment and high pressure operations are possible.
The desirability and efficiency of the plasma processes depend on many parameters such as the efficiency of electron excitation, the temperature, pressure and mix of gases that react with the electrons of the plasma to produce ions and radicals, the bias in the electric and magnetic fields that accelerate the ions and radicals onto the film, and the contaminating effects of plasma interactions with other surfaces in the plasma chamber such as the side walls of the plasma chamber. All these parameters can be varied almost continuously, so that extensive experimentation is required to determine what new combinations of parameters produce results that meet particular manufacturing design specifications. Such specifications include the selectivity for one film over another, the critical dimension, the aspect ratio of the resulting structures, the depth and steepness of the trench sides, i.e. the trench profile, and the etching rate.
As the complexity and integration of semiconductor devices continue to increase beyond LSI, limitations in the existing etching methods, such as low selectivity, poor anisotropy, micro-loading, and the difficulty in identifying new combinations of parameters with desirable results, inhibit the productivity of a fabrication facility. These problems are especially severe in the etching of metallic films for forming fine contact holes. The problems become worse as the number of different metallic layers in a metallic film increases.
The conventional processes for established semiconductor devices typically are not applicable to the new devices being designed and manufactured. As a result, the conventional etching processes cannot satisfy the modem trend toward highly-integrated semiconductor devices beyond LSI devices, and so these conventional etching processes limit the utility and reliability of fabrication facilities.